1. Field of the Invention
The invention is related to microelectromechanical systems (MEMS) and more particularly to manufacturing MEMS structures.
2. Description of the Related Art
In general, microelectromechanical systems (MEMS) are very small mechanical devices. Typical MEMS devices include sensors and actuators, which may be used in various applications, e.g., accelerometers, gyroscopes, and pressure sensors. The mechanical device is typically capable of some form of mechanical motion and is formed at the micro-scale using fabrication techniques similar to those utilized in the microelectronic industry, such as thin film deposition and thin film patterning by photolithography and reactive ion etching (RIE).
Certain MEMS devices include a resonator, which may be used in timing devices of an integrated circuit (IC). The resonator may have a variety of physical shapes, e.g., beams and plates. Referring to FIG. 1, a conventional MEMS device (e.g., MEMS device 100) includes a resonator (e.g., resonator 105) coupled to a substrate (e.g., substrate 102) via an anchor (e.g., anchor 104). During operation, a first electrode (e.g., electrode 110) electrostatically drives resonator 105 to dynamically deflect, which increases a capacitance of resonator 105 when a voltage differential exists between resonator 105 and electrode 110 by decreasing the gap between resonator 105 and electrode 110. Because electrode 110 and resonator 105 are the same height and in the same plane, resonator 105, when driven, deforms laterally across a distance between electrode 110 and a second electrode 111, remaining in plane 103 of the electrode 110. Plane 103 is substantially parallel to substrate 102. An electrode (e.g., electrode 111) detects the resonant frequency of resonator 105 as the capacitance varies between the two in response to the deflection driven by electrode 110. Because resonator 105 is driven to resonate in a mode where the resonator 105 remains in plane 103 of the electrode 110, the conventional MEMS device 100 is commonly referred to as an “in-plane” or “lateral” mode resonator.
There are several drawbacks to the parallel-plate-capacitor drive and sense mechanism of conventional MEMS device 100. The electrostatic force of MEMS device 110 is nonlinear unless the amplitude of vibration is limited to a small fraction of the capacitor gap. In addition, since the transduction efficiency of resonator 105 is dependent on the area of the parallel-plate capacitor formed between the resonator 105 and electrode 110, fabrication of the resonator 105 generally includes a number of techniques to ensure the resonator 105, when released, remains perfectly flat and in the plane of the electrode 110. Such fabrication techniques are often thermally taxing or require prohibitively expensive or commercially unfeasible methods.